Polypropylene to transportation fuel grade hydrocarbons over γ-alumina catalyst

Abstract

Catalytic upgrading of plastics to valuable fuels and chemicals is an attractive route to valorize waste plastics. Herein, catalytic pyrolysis of polypropylene was performed over γ-Al₂O₃ as a heterogeneous catalyst to produce fuel-grade hydrocarbons. The use of an inexpensive γ-Al₂O₃ catalyst and mild reaction conditions led to high liquid yield selectively in gasoline-range hydrocarbons which stands out from most of the work reported in the literature for polypropylene pyrolysis. The reaction conditions of pyrolysis were optimized by the Box-Behnken Design approach utilizing the response surface methodology. The highest liquid yield of 88.1 wt.% was obtained at 470 °C temperature, with 2 wt.% of catalysts and 5 h reaction time. The amount of solid carbon was insignificant (0.7 wt.%) and the gas yield was 11.2 wt.%. The γ-Al₂O₃ showed high efficiency and stability for converting polypropylene to liquid fuels. The catalyst was highly stable, reusable, and showed similar catalytic activity for 3 recycles. These features and the highly selective conversion of PP to gasoline range fuels are crucial for large-scale applications. The GC–MS analysis revealed that the liquid fuel produced mostly contained C8 to C15 hydrocarbons encompassing mostly gasoline and a small fraction of diesel fuel and higher hydrocarbons. The GC–MS data was also supported by SimDist analysis, which exhibited the boiling point ranging from 100 °C to 260 °C for the liquid fuel product. The reaction temperature and time had a significant impact on the liquid yield. The higher temperature favored the formation of the gaseous product of C1-C3 hydrocarbons. The NMR analysis showed that the liquid products mostly contained the highest amount of paraffins followed by olefins and a small fraction of aromatics. The presence of mild acidity in the γ-Al₂O₃ catalyst and optimum reaction condition provides favorable conditions to produce the highest yield of transportation fuel grade hydrocarbons without over-cracking into gases.